His close encounter with a Big Mac at a McDonald's restaurant in Washington is one more sign that despite a rocky year - marked by the disappointment of the Reykjavik summit, the arrest of an American journalist in Moscow, and mass expulsions of Soviet diplomats from the US - US-Soviet relations are far from foundering.

``Overall bilateral relations are still chugging along,'' one US official says. ``There's certainly been no slowdown.''

Nor was there at the McDonald's, where the Soviet teen-agers repaired after touring the Smithsonian's Air and Space Museum last week. Admittedly, they did have a bit of difficulty understanding the finer points of distinction between a ``Quarter-Pounder'' and ``Big Mac.'' But almost as soon as their orders were translated, they were handed trays laden with salads, hamburgers, French fries, and milk shakes.

It was the speed of the service that many of the youths found startling.

``Such quick service is not bad,'' said 15-year-old Pavel Kudryavtsev. ``We don't have such a system in the Soviet Union.'' A recent Soviet television report on McDonald's, he said, had suggested that perhaps the Soviet Union could learn something from such American efficiency.

``These sorts of exchanges,'' an American official says, ``help to open up the societies, one to another.''

And the opening seems to be widening, despite the failure to reach accord on nuclear arms control.

A high-level delegation from the Supreme Soviet - including the editor of Pravda, the official Communist Party newspaper - is now visiting Congress. The Soviet foreign trade minister was in Washington earlier in the month for talks with US officials. And 11 ``Young Cosmonauts'' who are destined to work in the Soviet space program are touring the US as guests of the Young Astronaut Council, a private American organization that promotes youth exchanges and improved scientific and technical education.

Some US officials are openly dismissive of such exchanges. One says they are ``essentialy peripheral'' to the central question of arms control.

But another official demurs, noting that in the absence of summits and major new agreements, ``these kinds of exchanges form the basis of our relationship.'' They also provide a counterpoint to the gloomy assessments of US-Soviet relations coming from some diplomats.

To be sure, the Soviets are irritated at the lack of progress on arms control.

``We have made a number of important proposals, and your government has not responded to them,'' one Soviet official says.

``We are growing very impatient,'' he said, adding that the Soviet Union was considering whether much could be accomplished during the final two years of the Reagan administration.

``They could essentialy pick up their marbles and go home,'' one US diplomat warns. But, the US official, that is not happening - yet.

A new agreement on US-Soviet space cooperation is nearing completion, and the two countries are considering 16 projects involving civilian space programs, according to a US official. The two governments are also considering ``some very promising areas of cooperation'' in nuclear fusion research, he adds.

Joint research projects in medicine and health care are also under consideration. The US surgeon general, C.Everett Koop, was in Moscow earlier this year to explore joint research projects.

The Supreme Soviet delegation, meanwhile, is exploring further parliamentary exchanges with Congress. One idea under consideration: a television ``space bridge'' between Moscow and Washington, linking lawmakers in both countries by satellite.

US officials have been impressed at the caliber of Soviet representatives that are being sent to this country, although they were somewhat disappointed that one key official - Georgy Kornienko, the first chief deputy head of the Central Committee's International Department - dropped out of the Supreme Soviet delegation just before it left for the US.

``The contacts continue,'' a US official says. ``They haven't suffered at all as a result of Reykjavik.'' And, he says, the ongoing exchanges indicate continuing interest in improving contacts.

``It seems to me that the Soviets are going to some length to give these visits a high profile. Perhaps to send a message that they're leaving a lot of options open.'' He adds, ``It's an interesting time in US-Soviet relations.''

That is an assessment with which Igor Novikov, one of the Soviet teen-agers now touring the US, would surely agree.

``I wanted to see life in the United States,'' he says. ``We have very many questions about it.''

Meanwhile, across the table, Pavel Kudryavtsev is bewildered by a plastic squeeze packet of salad dressing.

``Chto eto'' (What is this)? he asks.

A nearby American demonstrates the etiquette of opening it without using teeth or squirting clothes.